The expansion of renewable energies (REs) having fluctuating generation profiles that can only be controlled inefficiently poses increasing challenges for energy infrastructures. In addition to the grid expansion and approaches for load management, the provision of suitable storage capacities is required. In this case, not only high-dynamic stores for stabilizing power grids, but also technologies for taking up large excesses of REs come into consideration. In the latter case, currently, in addition to the established pumped-storage plants, especially compressed-air storage plants and chemical storage in the form of hydrogen are coming into consideration and pursued in research and development. In the case of hydrogen, in addition to the requirement for drastic decrease of capital costs with respect to components, and for increase in efficiency in the conversion steps, there is the question of a suitable infrastructure for storage, distribution and use of the hydrogen.
In recent years, there have been numerous approaches for a hydrogen infrastructure. A differentiation must be made between the approaches in which hydrogen is stored, transported and utilized separately. A disadvantage in this case is the considerable expenditure for introducing a separate infrastructure. Especially the example of hydrogen mobility shows that there are more than insignificant obstacles during introduction, if such concepts are based on widespread availability of the energy carrier. Also, the frequently favoured utilization of central subterranean cavern stores for the storage of hydrogen means a considerable entry hurdle, owing to the high initial capital cost. Not least, it would seem to be necessary for the users to install new equipment.
Consequently, there has been much work concerned with utilizing the existing natural gas infrastructures for storage and transport of regeneratively generated hydrogen. In the context of the EU-funded project NaturalHy, the technical and in particular safety-related feasibility of feeding hydrogen into natural gas pipelines has been studied and demonstrated. Relatively high, and especially fluctuating, fractions of hydrogen in the natural gas, however, lead to problems in the end equipment, since much end equipment only permits a comparatively narrow range in what is termed the Wobbe index and a broadening of the range would cause complex additional installations on the end equipment. The Wobbe index of a flammable gas as specified in DIN 51857 is calculated from the heating value and the relative density of the gas and describes the heat output achieved in the combustion of the gas in a nozzle burner. In the Netherlands, for example, the Wobbe index must be maintained within a range of approximately 1 MJ/m3. However, even at low fractions of approximately 5% by volume the addition of hydrogen leads to departure from the range. Direct feeding of hydrogen into conventional gas grids which are used, for example, for distribution of natural gas, is therefore possible only to a limited extent.
An alternative possibility for the practical use of relatively large amounts of power originating from REs is the conversion of hydrogen and carbon monoxide or carbon dioxide to methane which hereinafter will also be termed methanation. In this case, there is in principle no restriction with respect to the amount of power fed in. On the other hand, it must be noted that methanation, in addition to hydrogen generation, requires a considerable expenditure in terms of apparatus and is accompanied by further energy losses. For instance, methanation proceeds in a gas-phase process at about 200 to 300° C. at excess pressure. The reaction is equilibrium-limited, and so a more or less extensive work-up and cycling of the process gases is required. In addition, the reaction is highly exothermic. The heat of the reaction generally—in particular in the case of relatively large plants—cannot be utilized, or can be utilized only in part. Therefore, the in any case already low efficiency is further reduced in the overall conversion chain.